Cellulose ethers such as methyl cellulose and hydroxypropyl methyl cellulose have such thermoreversible gelation properties as to undergo gelation during heating, and are used in processed foods to improve the shape retention of heated foods by taking advantage of the thermoreversible gelation properties.
The methyl cellulose has a higher thermal gel strength than that of the hydroxypropyl methyl cellulose, but a thermal gelation temperature of a 2% by weight aqueous solution of the methyl cellulose is as low as about 50 to 60° C. so that the methyl cellulose is still in a gel state at the food temperature of 50 to 70° C. during eating, and has a possibility of providing hard texture. In addition, the methyl cellulose has to be once cooled at 15° C. or lower for complete dissolution in preparation of a solution thereof, thereby making the preparation complicated.
The hydroxypropyl methyl cellulose has a relatively higher thermal gelation temperature of 61 to 80° C. than that of the methyl cellulose, and the thermal gelation temperature can be adjusted to 70° C. or higher by controlling the substitution degree. Thus, the hydroxypropyl methyl cellulose has a lower possibility of providing hard texture. Since the hydroxypropyl methyl cellulose can be dissolved at 25° C. or higher, no cooling operation is required in preparation of a solution thereof, thereby making the preparation simple. For these reasons, the hydroxypropyl methyl cellulose having a high thermal gelation temperature is preferred to improve the shape retention of a food during heating in absence of providing hard texture.
When added to a food, hydroxypropyl methyl cellulose having a low viscosity is preferred from the standpoint of operability. Since the thermal gel strength of hydroxypropyl methyl cellulose is variable greatly depending on the viscosity thereof, however, high shape retention requires a high viscosity, thereby resulting in poor operability. On this account, there is a demand for the hydroxypropyl methyl cellulose having a high thermal gel strength even at a low viscosity.
As the method for producing hydroxypropyl methyl cellulose having a high thermal gel strength, a production method in which a cellulose is alkalized in at least two steps as described in JP 2013-539815T, which is a Japanese phase publication of WO 2012/051035, and a production method in which a methyl etherifying agent is reacted prior to a hydroxypropyl etherifying agent as described in JP 2011-144350A are known.